SEOUL, April 23 (Reuters) - U.S. activist hedge fund Elliott Management dismissed Hyundai Motor Group's restructuring plan as insufficient on Monday and called on the South Korean conglomerate to adopt a holding company strategy and appoint more independent board members.

Elliott disclosed this month that it holds more than $1 billion worth of shares in three key affiliates of Hyundai Motor Group and called for a "more detailed roadmap" on how the group will improve corporate governance, optimize balance sheets and enhance capital returns.

Stepping up its campaign against the South Korean carmaker on Monday, Elliott said it was unclear how the group's restructuring plan would benefit minority shareholders.

The giant autos-to-steel group last month announced a plan to streamline its ownership structure, responding to calls from the government and investors for greater transparency and better governance at family-controlled conglomerates, or chaebols.

"Elliott is encouraged that the Group has acknowledged the need for an improved ownership structure. However, the unwinding of the current circular shareholding by itself... lacks clear benefits to minority shareholders," Elliott said in a statement.

It said it holds more than 1.5 percent of the common shares in each of the three key affiliates of the country's No.2 conglomerate - Hyundai Mobis, Hyundai Motor and Kia Motors.

Hyundai said in a statement that it would continue to communicate with investors including Elliott to explain the goals and needs of its reorganization plan.

It is Elliott's latest challenge to South Korea's family-run conglomerates after it forced Samsung Electronics Co Ltd to increase shareholder returns in 2017, and comes amid a government campaign to boost investors' power in a country where shareholder activism is rare.

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Elliott said the Hyundai group should combine Hyundai Mobis with Hyundai Motor to create a holding company, and set up a clear policy for dividend payouts.

"We believe this is truly a unique opportunity to unlock significant value in Hyundai Motor Group that the founding family and the leadership of the Group have built over the decades," Elliott said in its presentation.

In its introduction, Elliott boasted of its "successful" 2016/17 campaign against Samsung Electronics, in which it also urged a restructuring of the business as a holding company. Samsung turned down Elliott's push for the holding company structure, but instead boosted dividends and cancelled treasury shares.

Regardless of whether Hyundai adopts a holding company structure, Elliott demanded the cancellation of treasury shares in Hyundai Motor and Hyundai Mobis.

Elliott said Hyundai Mobis and Hyundai Motor each has as much as 6 trillion won ($5.6 billion) of excess cash, but they "have used valuable capital to fund many questionable investments" including the group's $10 billion bid to buy a plot of land in Seoul at a 217 percent premium over the land's appraised value.

"We believe that Hyundai Motor Group’s current balance sheet is out of balance," it said.

Elliott also proposed that the three affiliates add three independent board members each to improve the group's corporate governance, which it said "remains below global standards."

Kim Sang-jo, head of South Korea's anti-trust watchdog, said on Friday that he expected talks between Hyundai and Elliott would not be friendly, although Hyundai may have prepared for the U.S. fund coming to challenge it. ($1 = 1,076.3300 won) (Reporting by Ju-min Park and Hyunjoo Jin; Editing by Muralikumar Anantharaman and Susan Fenton)